The invention relates generally to a compact, efficient microwave electron tube. More particularly, the invention is a device capable of achieving high power at frequencies greater than 10 GHz.
There are a number of applications requiring a high frequency, high power microwave source, including, for example, a linac collider used in elementary particle physics research, aerospace communications, radar, military electronic counter-measures, military electronic warfare, and plasma heating for nuclear fusion (electron cyclotron resonance). Various technologies are currently being explored to provide a microwave source that operates at frequencies between 10 and 30 GHz, with a drive power of at least 100 Mw To date, however, no such microwave source is available.
Microwave tubes are commonly used in a variety of communication systems, radar systems, and heating systems. The two principal types of conventional microwave electron tube amplifiers include the klystron and the traveling wave tube. Referring now to FIG. 1, a prior art klystron microwave amplifier includes a cathode, a buncher cavity, a catcher or output cavity, and a collector. In the klystron, a dc electron beam is generated at the cathode and transmitted through the cavity gaps and through a cylindrical metal tube between the gaps. The area in the metal tubes defines a "drift space" for the electron beam, and each of the cavities is essentially a resonator. A low-level rf input signal is coupled to the first resonator, which is called the buncher cavity. A waveguide or a coaxial connection may be used to couple the rf signal to the buncher cavity. The cavity is tuned to the frequency of the rf input signal and is excited into oscillation. As a result, an electric field exists across the buncher gap, alternating at the input frequency. For half a cycle, the polarity of the electric field is such that it causes the velocity of the electrons flowing through the cavity gap to increase. During the other half cycle, the polarity of the electric field is such that it causes the electron velocity to decrease. For example, when the voltage across the gap is negative, the electrons may decelerate; when the voltage is zero, the electrons are unaffected; when the voltage is positive, the electrons are accelerated.
Referring now to FIG. 2, after leaving the buncher cavity the electrons proceed through the tube's drift region. In the drift region, the electrons that were accelerated through the buncher gap will overtake electrons that were decelerated. Consequently, the electrons begin to form into bunches by the time they reach the gap of the second resonator, which is called the catcher cavity. The time between the arrival of the sequential electron bunches is approximately the same as the period of the rf input signal.
According to conventional techniques, the initial bunch of electrons flowing through the catcher cavity (also sometimes called the output cavity) will cause the cavity to oscillate at its resonant frequency. As a result, an alternating electric field E oscillating at the desired frequency E=E.sub.o cos (.omega.t+.phi.) is generated across the catcher cavity gap, where E.sub.o is the amplitude of the electric field, .omega. is the angular frequency of the field, t is time, and .phi. is the phase of the field. With proper design of the catcher cavity, the resonant frequency is such that each succeeding electron bunch that arrives at the catcher cavity gap encounters a decelerating electric field E that extracts most of the energy from the electrons, where E.sub.g =eV (g is the size of the gap; eV is the electron energy). If, however, electrons are out of phase and arrive at the catcher cavity when the polarity of the electric field across the gap is reversed, the electrons are accelerated, and energy is removed from the catcher cavity. In an effort to improve the bunching process, klystrons may be provided with intermediate cavities.
Some of the general limitations to the design of conventional klystron electron tubes are their large size, high operating voltages, and the complexity of associated equipment, such as that required for cooling and for providing magnetic guide fields.
Referring now to FIG. 3, a traveling wave tube includes a cathode, an anode, an input coupler, an external helix circuit, and an output coupler. The cathode produces a stream of electrons in a known configuration. The electrons are focused and confined into a narrow beam by an axial magnetic field, such as an electromagnet (not shown) which surrounds the helix portion of the tube. As the electrons pass through the helix, the narrow beam is accelerated by a high electric potential on the helix and the collector.
The beam in the traveling wave tube continually interacts with a varying electric field, emanating from an rf wave that propagates along an external circuit surrounding the beam. The propagating rf wave is generated from the longitudinal components of an rf signal received at the input coupler. To achieve amplification, the rf wave propagating on the external circuit has a phase velocity that is nearly synchronized with the velocity of the electron beam. Because it is difficult to accelerate the electron beam to more than about one-fifth the velocity of light, the forward velocity of the rf field propagating along the external circuit must be reduced to nearly that of the beam.
The phase velocity in a waveguide, which is uniform in the direction of propagation, is always greater than the velocity of light. However, this velocity may be reduced below the velocity of light by introducing a periodic variation of the circuit in the direction of propagation. The simplest form of variation is obtained by wrapping the circuit in the form of a helix which acts as a "slow wave" structure.
As explained previously, the electron beam is focused and constrained to flow along the axis of the helix. The longitudinal components of the input signal's rf electric field, along the axis of the helix or slow wave structure, continually interact with the electron beam to provide the amplification of the traveling wave tube.
If the electron beam velocity is exactly synchronized with the circuit's phase velocity, the electrons experience a steady electric force which tends to bunch them. In this case, as many electrons are accelerated as are decelerated; hence there is no net energy transfer between the beam and the rf electric field. To achieve amplification, the electron beam is adjusted to travel slightly faster than the rf electric field propagating along the helix. The bunching and debunching mechanisms are still at work, but the bunches now move slightly ahead of the fields on the helix. Under these conditions more electrons are decelerated than are accelerated, and energy is transferred from the beam to the rf field.
The fields may propagate in either direction along the helix. This leads to the possibility of oscillation due to the reflections back along the helix. This tendency is minimized by placing some resistive material near the input end of the slow wave structure. This resistance may take the form of a lossy attenuator to absorb any backward traveling wave. The forward wave is also absorbed to a great extent, but the signal is carried past the attenuator by the bunches of electrons. Since these bunches are not affected as they pass by the attenuator, they are capable of reinstituting the signal on the helix.
The rf signal may be removed from the traveling wave tube by a coaxial cable as in FIG. 3 or by a number of other ways, as shown in U.S. Pat. No. 4,682,076 issued to Kageyama, et al., U.S. Pat. No. 4,147,956 issued to Horeqome, et al., and U.S. Pat. No. 4,658,183, issued to Huber.
Both the klystron and the traveling wave coupler utilize a round dc electron beam. An rf wave is developed on the dc electron beam and the beam is then coupled to the modulated current in an output coupler. As a result, the klystron, the traveling wave tube and similar devices encounter fundamental performance limitations as frequency and power both increase, due primarily to the round beam geometry and the requirement of modulating a dc beam. The round beam geometry results in a space charge depression in the center of the beam. Space charge refers to the depression of the beam energy in the interior of a round beam compared to the energy of the electrons in the exterior part of the beam. The net result is that the interior of the round beam travels more slowly than does the exterior, producing a transit time spread in the acceleration and drift of the beam. The amount of space charge depression is proportional to beam current.
In high power applications, a high beam current magnifies the problem with space charge depression and causes a phase dispersion or phase spread in the electron bunch. In addition, as the frequency of the rf field increases (and the spacing between the electron bunches decreases), the effect of the phase spread becomes more significant. Consequently, as beam power and frequency increase, the electrons no longer bunch satisfactorily. Both the klystron and traveling wave tube have upper limits of about a few kilowatts of power at about 10 GHz.
Another problem with klystrons and traveling wave tubes occurs at high power and high frequency. Increasing beam current to achieve high power requires that beam size be increased, and as beam size increases, the input and output couplers must also be larger. As the beam size and the size of the couplers approach the wavelength of the rf signal, the coupler may have both a decelerating electric field and an accelerating electric field simultaneously and thus the phase of the coupling is different across the dimension of the beam. Consequently, extracting energy from the beam becomes far less efficient for a large beam size.
A final problem arises from the use of magnetic guide fields in the klystron and traveling wave tube. In conventional electron tubes, a magnetic field is used to guide the electron beam after it leaves the cathode. As the beam size increases, so too must the magnetic field in order to guide the larger beam. There is ultimately a limit to the beam size that can be successfully guided. In addition, the necessity of using magnets to guide the electron beam substantially increases the size and weight of the electron tube.
Several devices, including the gyroklystron, free-electron laser and the lasertron were developed recently in an effort to eliminate some of these inherent limitations to microwave amplifier performance. The prior art lasertron is a round beam device and encounters significant limitations at high power and high frequency due to space charge effects. The lasertron uses either a photocathode or a field emitter brush which is excited by a modulated laser beam. The photocathode generates a round electron beam with all of its inherent limitations. GaAs photocathodes have been used on several prototype lasertrons, but deteriorate in frequency response above 1 GHz, because the charge mobility is inadequate to clear the depletion depth during an rf cycle. Further, the modulated laser and photocathode have significant uncertainties rendering these units inappropriate for reliable continuous operation in most applications. Example of lasertrons are disclosed in U.S. Pat. No. 4,313,072, issued to Wilson, et al; C. K. Sinclair, SLAC-PUB-4111 (1986); Y. Fukushima, et al., Nucl. Instr. Meth. A238, 215 (1985); and R. B. Palmer, SLAC-PUB-3890 (1986).
A free electron laser requires extremely high beam brightness for efficient operation and requires a formidable infrastructure of accelerator equipment. An example of a free electron laser is disclosed in U.S. Pat. No. 4,571,726, issued to Wortman, et al, and in A. M. Sessler and D. B. Hopkins, LBL-21618 (1986).
The gyrotron and gyroklystron couple to gyromotion of an electron beam, and are capable of generating megawatts of output power at frequencies greater than 10 GHz. The gyrotron operates as an oscillator, while the gyroklystron is stabilized to operate as an amplifier. Both devices suffer from intrinsic phase instability for applications requiring phase control. In addition, both devices require large, complex electromagnets to provide an appropriate guide field, and both devices are sensitive to variations in load impedance. An example of a gyroklystron may be found in W. Lawson et al., "A High Peak Power, X-Band Gyroklystron for Linear Accelerators," Proc. 1986 Linac Conference.
To date, no one has successfully developed a compact, efficient microwave source to operate at a frequency above 10 GHz with approximately 100 MW.